A general kinetic treatment of the system with intermolecular chain transfer followed by fast reinitiation is given. It leads to the broadening of the molecular weight distribution (MWD), the number of growing chains being invariable. Thus, this system can be considered as a special case of living polymerization. A general method has been elaborated allowing the determination of the ratio of the rate constant of propagation ($) to the rate constant of the bimolecular transfer ( k f ) ) from the dependence of the MWD on monomer conversion. Numerical values of k,/kf) equal to =lo2 and 25 were thus determined for the polymerization of L,L-lactide (L,L-dilactide) initiated with aluminium tris(isopropoxide) trimer ({ Al(O'Pr),) 3) and tributyltin ethoxide ("Bu$nOEt), respectively.
A computational study of the branching in polyacrylates is performed for both atom-transfer radical polymerization (ATRP) and free radical polymerization (FRP). In both cases the secondary radical formed can transfer to polymer to generate a tertiary radical, which can propagate with monomer to re-form the secondary species. The critical difference between these two processes is that the exchange between tertiary and secondary species is supplemented in ATRP by additional activation and deactivation reactions for both the secondary and tertiary species. This leads to a competition between the activation–deactivation and exchange processes in ATRP, while there is no such competition in FRP. This introduces the idea of competing processes or equilibria. These competing processes can alter the fate of the tertiary radical in ATRP, by introducing a deactivation step, in addition to the propagation, or branch formation, available in FRP. Various simulations show that, in order to effectively decrease the branching fraction in ATRP, the tertiary radical must be deactivated relatively rapidly. Then, the rate of branch formation is slower than the rate of transfer, resulting in a decrease in the branching fraction. Kinetic simulations also find that concentrations of copper catalysts have minimal effect on the branching fractions and that higher initiator concentrations tend to decrease the branching levels in ATRP. Furthermore, Monte Carlo simulations found that chain length dependence and presence or absence of intermolecular transfer had minimal effect on the branching fraction.
Chain transfer processes (ktr) taking place in the polymerization (anionic and pseudoanionic) of cyclic esters (lactones) are reviewed. These reactions are mostly responsible for the departure of these systems from the fully controlled (living) polymerizations. The ratios of kp/ktr have been determined for a number of initiating systems and the structures of the growing species are related to their selectivity, expressed by kp/ktr. It has been shown that the less reactive and more sterically crowded active species polymerize more selectively.
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